The fundamental maths of fat loss
Fat loss requires a calorie deficit — consuming fewer calories than you expend. One kilogram of body fat stores approximately 7,700 kcal of energy. A daily deficit of 500 kcal produces roughly 0.5 kg of fat loss per week in theory. In practice, the relationship is slightly less linear because the body adapts to restriction, but this is a reliable starting estimate.
The key word is fat loss, not weight loss. Scale weight includes water, muscle, and food in transit as well as fat. Body composition change — losing fat while preserving or building muscle — is determined by deficit size, protein intake, and resistance training together. Focusing only on the calorie number misses two of the three variables that determine how you look and feel after a diet.
Calculate your TDEE first — your deficit is set relative to how many calories you burn daily.
TDEE Calculator →How to choose the right deficit size
The right deficit depends on four factors: your current body fat level, your hunger tolerance, your training history, and how long you plan to diet.
Deficit size by starting body fat
Body fat level is the most important factor in determining how large a deficit you can sustain without losing muscle. When fat stores are abundant, the body preferentially burns fat during a deficit. As you get leaner, it becomes increasingly protective of fat reserves and more willing to sacrifice muscle protein for energy — which is why the appropriate deficit size decreases as you lose weight.
| Body fat level | Recommended deficit | Expected weekly loss | Notes |
|---|---|---|---|
| High (>30% F / >25% M) | 500–750 kcal/day | 0.5–0.75 kg | Larger deficit sustainable; muscle loss risk lower |
| Moderate (20–30% F / 15–25% M) | 300–500 kcal/day | 0.3–0.5 kg | Standard range; most people here |
| Lean (<20% F / <15% M) | 200–300 kcal/day | 0.2–0.3 kg | Small deficit; high protein critical |
| Very lean (<15% F / <10% M) | 100–200 kcal/day | 0.1–0.2 kg | Recomposition territory; very slow progress |
The case for a moderate deficit (300–500 kcal)
This is the evidence-based sweet spot for most people. A 2020 systematic review in Obesity Reviews found that moderate deficits (resulting in 0.5–1% of bodyweight loss per week) produced better body composition outcomes than aggressive deficits — losing more fat relative to muscle. The reasons are well-established:
- Sufficient calories to support training performance and muscle protein synthesis
- Less cortisol elevation, which at high levels promotes muscle breakdown and fat storage
- Less severe hunger, making adherence more likely
- Less metabolic adaptation (the reduction in TDEE that occurs with aggressive restriction)
When a larger deficit (500–750 kcal) is appropriate
A larger deficit is appropriate when starting body fat is high, when there's significant weight to lose, and when the psychological urgency of faster progress supports adherence. The key safeguard at larger deficits is protein intake — maintaining 1.8–2.4g/kg of bodyweight protects muscle mass even in significant deficits for people with adequate fat stores.
The 1% rule
A useful rule of thumb: aim to lose no more than 1% of bodyweight per week. For a 90 kg person, that's 0.9 kg per week, requiring a deficit of approximately 700 kcal/day. For an 70 kg person, 0.7 kg per week needs around 500 kcal/day. Going faster than 1% per week significantly increases the proportion of weight lost as muscle rather than fat.
When deficits get too large
Very large calorie deficits (above 1,000 kcal/day, or very low calorie diets below 800 kcal/day) produce faster weight loss in the short term but cause problems that worsen long-term outcomes.
Muscle loss
In aggressive deficits, the proportion of weight lost as lean mass increases substantially. A 2021 meta-analysis in the British Journal of Nutrition found that dietary interventions producing more than 1.2% of bodyweight loss per week resulted in significantly more lean mass loss relative to fat, regardless of protein intake. Lean mass loss reduces metabolic rate and makes weight regain more likely after the diet ends.
Metabolic adaptation
The body responds to large deficits by reducing energy expenditure through multiple mechanisms: decreased non-exercise activity thermogenesis (NEAT — fidgeting, spontaneous movement), reduced thyroid output, lower reproductive hormone levels, and improved metabolic efficiency. This adaptation can reduce TDEE by 200–500 kcal below what weight alone would predict — progressively shrinking the actual deficit even as food intake stays the same.
Hormonal disruption
Very low calorie intake suppresses leptin (the satiety hormone), elevates ghrelin (the hunger hormone), and reduces testosterone and oestrogen. These changes increase hunger, reduce recovery from training, impair sleep quality, and in women can cause menstrual disruption. These effects are more severe the leaner you already are and the larger the deficit.
The variable that matters as much as deficit size: protein
Protein intake during a deficit is as important as the deficit size itself for determining body composition outcomes. Research consistently shows that high protein intake (1.8–2.4g/kg) at the same calorie deficit produces dramatically better muscle retention than lower protein intake — sometimes by a factor of 2–3 in the ratio of fat lost to muscle lost.
If you take one thing from this article beyond the deficit number: set your protein target first, then build the rest of your calorie intake around it. Protein preserves muscle, increases satiety, raises TDEE through the thermic effect of food, and makes the same deficit feel more manageable.
Calculate your protein target — as important as your calorie deficit for body composition.
Protein Calculator →How to adjust your deficit over time
Calorie needs change as you lose weight. Your TDEE decreases as your body becomes lighter, and metabolic adaptation further reduces it. A deficit that was accurate at the start of a diet typically produces less fat loss after 8–12 weeks — not because something has gone wrong, but because the maths have changed.
Recalculate TDEE every 4–6 weeks
Recalculate your TDEE at your current (lower) bodyweight every 4–6 weeks and adjust your calorie target accordingly. This keeps your actual deficit in the intended range as you lose weight.
Diet breaks for extended diets
For dieting phases longer than 12–16 weeks, incorporating planned diet breaks (2–4 weeks eating at maintenance calories) reduces the depth of metabolic adaptation and can improve total fat loss compared to continuous restriction over the same period. A 2020 study in the International Journal of Obesity (the MATADOR study) found that intermittent energy restriction with diet breaks produced 47% more fat loss than continuous restriction at the same total calorie deficit.
Refeed days
Shorter than a full diet break, a refeed day raises calories to maintenance (or above) for 1–2 days. Refeeds temporarily restore leptin, reduce cortisol, and provide psychological relief from restriction. They are most beneficial for people who are already lean and experiencing significant hunger or metabolic adaptation.
Practical deficit strategies that work
Create the deficit primarily through food, not exercise
Exercise is a less reliable deficit-creation tool than most people expect, for two reasons: most people overestimate how many calories exercise burns, and exercise-induced hunger often partially compensates for the calories burned. A 500 kcal deficit through reduced food intake is more reliable than trying to create 500 kcal through exercise.
Use walking to increase TDEE
Walking is the exception. It burns meaningful calories (250–400 kcal per hour at a brisk pace), produces very little compensatory hunger, and doesn't impair recovery from resistance training. Adding 7,000–10,000 steps daily can create or expand a calorie deficit without the appetite compensation that higher-intensity exercise often triggers.
Track for accuracy
Studies show people underestimate their food intake by an average of 30–50% without tracking. A few weeks of weighing food (not estimating) reveals the actual calorie content of your diet — which is often very different from what you think. This isn't about tracking forever; it's about building an accurate mental model.